Rain Catchers

Most plants acquire water through their belowground roots, but Pandanus forsteri—an unusually tall screwpine endemic to Lord Howe Island off southeast Australia—sits on a tepee of aboveground roots. Some strands within its finger-like clusters of aerial prop roots—offshoots from a central stem—can take years to reach the ground. New research demonstrates P. forsteri’s evolutionary strategy for catching rain so its aerial roots get water without soil contact.

Matthew Biddick, Ian Hutton, and Kevin Burns from Victoria University in Wellington, New Zealand, examined how the structure of P. forsteri’s leaves and roots help it passively harvest rainwater. They first tested whether the grooves that run the length of its long leaves act like rain gutters. From measuring the length, width, and depth of these grooves, they found that the leaves are more gutter-like closer to the trees’ trunks. Furthermore, when the researchers covered the leaf grooves with tape, the leaves collected significantly less water compared to when they were unaltered.

The researchers also tested how P. forsteri’s roots conduct, transport, and store water. When channels in the roots were tape-disabled, they collected less water, especially roots at shallower angles. Furthermore, spongy tissue at the tips of aerial roots was found to aid in water retention, holding nearly ten times its dry mass when soaked in water, compared to regular root tissue, which held only 1.5 times its dry mass.

Finally, the researchers tested how manipulating the root channels or removing aerial root tissue affected root growth. Unaltered trees, followed for twenty-two months, were compared with those with root channels disabled by silicone collars, or with water-retaining root tissue surgically removed. Both manipulations resulted in stunted growth—compared to unaltered controls, roots with disabled channels were shorter, while roots with removed tissue were thinner.

P. forsteri leaves, trunks, and roots act as a passive water transport system, like Roman aqueducts or the eaves of modern houses. Says Biddick, channels in leaves and roots “are like the downspouts and the spongy [root tip] tissue is like the storage unit.” These adaptations may be what allows it to grow more than three times the size of most other screwpine species. (Proceedings of the Royal Society B)